The present invention relates to a dc power unit which is suitable to an information processing apparatus such as a connecting unit between LANs, and more particularly to a technique which may be effectively applied to the dc power unit for coping with regulations for harmonics through the effect of a boosting two-converter system.
In order to take measures for regulations for harmonics required by a dc power unit used for an information processing apparatus such as a connecting unit between LANs, as a technique for suppressing harmonics of a power unit, there have been proposed a choke input rectifying type, a current injecting type, a one-converter type based on a Mag-Switch type, and a boosting two-converter type. These prior arts have been published in the writing: "Electronic Technology", March 1995 (Vol. 37, No. 3, 1995), pages 36 to 41 and 96 to 97. These techniques have their own merits and demerits. Of them, today, the most remarkable and the most frequently realized technique is the boosting two-converter type.
With reference to FIG. 8, the circuit of the boosting two-converter type dc power unit will be described below. FIG. 8 illustrates a switching transistor 5, a control circuit 17 located around the switching transistor 5, and an insulating dc-to-dc converter 8 located at a later stage. The control circuit 17 receives an input voltage waveform applied thereto as a reference current Iref. At first, the reference current Iref is compared with a drain current Id sensed by a resistor 6. If the drain current Id exceeds the reference current Iref, a comparator 12 produces an inverted output and serves to turn off the switching transistor 5. As shown in FIG. 10, therefore, the drain current flows on a reference input voltage.
This current Id gives rise to energy in a coil 3. When the switching transistor 5 is turned off, at a time, counter electromotive force is generated in the coil 3, so that the coil 3 is served to invert its polarity and discharge the energy stored therein to a capacitor 7 through a diode 4. The capacitor 7 operates to smoothen the energy current into dc current. The rise of pulsating current results in boosting the reference current Iref and thereby enlarging the drain current Id (see FIG. 10). This inverts the comparator 12 so that the amount of energy stored in the coil 3 and conveyed from the coil 3 to the output is made larger accordingly.
The repetition of the foregoing operation yields a dc electric power at the output. If the pulsating current is applied onto the input side, since a maximum value of current at each period is on a sine curve, the smoothed pulsating current is made to be a sine waveform that is equal to an input voltage waveform. It means that, in principle, a power factor of 1 can be obtained. Hence, no power harmonics are generated. Practically, a power factor of 0.9 or higher can be easily obtained. If well-designed and adjusted, it is possible to realize a power factor of 0.99.
Apart from the capacitor input rectifying type, no smoothing capacitor is connected immediately after the rectifier. No pulsating current is flown through the input line. This results in being able to produce a high power factor and suppressing harmonics of a power unit.
The aforementioned arrangement makes it possible to produce a stabilized output. By changing the reference current value Iref, the conducting period of the switching transistor 5 may be changed. It indicates the output voltage may be controlled.
That is, if some factor lowers the output voltage, an error amplifier 10 operates to increase the reference current Iref. This operation makes it possible to extend the ON time of the switching transistor 5 at each period, raise the Id value and thereby the output voltage.
The voltage precision of the dc output voltage Vo is determined by the reference voltage Vref and the gain of the error amplifier 10. Hence, the combination of the control circuit 17 with an insulating dc-to-dc converter 8 results in suppressing the harmonics and constructing a wide-range continuous variable output power unit ranging from 90 to 264 ac volts of a power input.
However, if the conventional boosting two-converter type power unit is powered up when the low power is applied to the input (the 100-volt system power from 90 to 120 ac volts is input), the power output Vo1 is overshot, so that an excessive voltage sensed by the protecting circuit may turn off the dc power unit.
This phenomenon will be described with reference to FIGS. 8 and 9. When the input switch 16 is turned on, the ac input is applied to the rectifying diode 2. Then, the rectifying diode 2 operates to output a full-wave rectified voltage (127 to 180 dc volts) to a point Vo.
When the boosting converter is softly started (an on/off pulse width of the switching transistor 5 is gradually changed), as shown by a waveform of Vo shown in FIG. 9, Vo is boosted to the set voltage in one stroke.
The set voltage Vo is set to be a higher voltage than the rectified voltage (360 dc volts) of 240 V+6%=25.44 ac volts that is high at the ac input 1. In general, it is about 380 dc volts.
Hence, the abruptly changing voltage shown by Vo of FIG. 9 is applied to the insulating dc-to-dc converter 8. The dc-to-dc converter operating at 380 dc volts normally departs from the control range if the input voltage ranges from 127 to 180 dc volts and disables to output a rated voltage (for example, +5 volts). The converter in this state is operating in the state that the control pulse (for example, +2 to +3 volts) is opened to a maximum limit. At this time, the abrupt rise of Vo delays a transient response of a feed-back system, thereby causing the overshoot phenomenon in the power unit Vo1. In the worst case, the power output Vo1 may be sensed as an excessive voltage by the protecting circuit. This may result in turning off the operation of the converter.